1. Field of Search
This invention relates to a selective reagent and method for aromatic nitration. More particularly this invention relates to a method of aromatic nitration by use of a reagent which is a complex of a nitronium containing substance and a polyether, which method exhibits high positional and substrate selectivity.
2. Prior Art
The nitration of aromatic hydrocarbons is an important industrial process. Of particular importance is the nitration of toluene for the production of toluene diisocyanates.
A common aromatic nitration is the reaction of toluene with HNO.sub.3 /H.sub.2 SO.sub.4 /H.sub.2 O to produce a mixture of isomeric nitrotoluenes having the following composition: 33-34% p-nitrotoluene, 3-4% m-nitrotoluene and 62-63% o-nitrotoluene. Separation of the mixture into its components entails complicated physical methods. See, Kirk-Othmer, Encyclopedia of Chemical Technology, Anthony Stander, Ed., Vol. 13, p. 845, 2nd Edition, Interscience Publishers, New York, N.Y.
A large body of literature is directed to efforts to alter the positional and substrate selectivity in aromatic nitration ("Industrial and Laboratory Nitrations", L. F. Albright and C. Hanson, Eds., ACS Symposium Series, Vol. 22, American Chemical Society, Washington, D.C., 1976).
Aromatic hydrocarbons, e.g., benzene, toluene are commonly nitrated by mixtures of nitric and sulfuric acid. A variety of evidence indicates that the nitronium ion, NO.sub.2.sup.+, is formed and is the actual nitrating agent. (Ronald Breslow, Organic Reaction Mechanisms, W. A. Benjamin, Inc., 2nd Ed. of New York, N.Y. 1969, p. 148).
A variety of nitronium containing nitrating agents e.g., NO.sub.2 BF.sub.4, NO.sub.2 PF.sub.6 and NO.sub.2 [CF.sub.3 SO.sub.3 ] have been used to alter the radio of ortho to para substitution and the amount of meta product. (C. L. Coon, W. G. Blucher and M. E. Hill, J. Org. Chem., Vol. 38, No. 25, p. 4243-4448 (1973); G. A. Olah, "Fundamental Study of Toluene Nitration", U.S. NTIS, AD-AO21-145, 1976). However, reduction in the amount of meta substitution to a value below 2% of the total mixture of mononitration products was usually effected by lowering the reaction temperature. It would be desirable to mimick this temperature effect by chemical means.
A comprehensive survey of a large number of methods of mononitration of toluene has been made by Olah (Fundamental Study of Toluene Nitration NTIS, AD-AO21-145). While some of the reagents surveyed were complexes of various nitronium containing substances, e.g., NO.sub.2 PF.sub.6, NO.sub.2 BF.sub.4, etc., with alcohols, dialkyl ethers, dialkyl sulfide, tetrahydrofuran and substituted pyridines, the percentage of meta nitrotoluene in the product mixture generally exceeded 3%. None of the reagents surveyed combined positional and substrate selectivity.
Macrocyclic polyethers, e.g., crown ethers are known to complex metal cations, especially alkali, alkaline earth and various transition metal cations and thereby enhance the reactivity of the counter anions. U.S. Pat. Nos. 3,562,295 (C. J. Pedersen) and 3,997,563 (J. Dale et al.). The use of crown ethers as phase-transfer catalysts is disclosed by E. V. Dehmlow in Angew. Chem. Int. Ed. Engl., Vol. 16, No. 8, at page 494 (1977).
The complexation of lithium, sodium and potassium ions by dimethyl ethers of polyethylene glycols is disclosed by L. L. Chan et al. in The Journal of The American Chemical Soc., Volume 92, No. 7 at page 1955 (1970). None of these references discloses an aromatic nitrating agent which is substrate selective and effectively reduces the amount of meta substitution.
It is accordingly an object of the present invention to provide a process for the selective nitration of aromatic hydrocarbons at ambient temperatures.
It is another object of the present invention to provide a process for the mononitration of aromatic hydrocarbons with minimal meta substitution.
It is still further an object of the present invention to provide an aromatic nitrating agent which is positional and substrate selective.